Interference layer system

ABSTRACT

A process is provided for sputter-induced precipitation of metal oxide layers on substrates by means of a reactive sputter process. The plasma charge acting upon the target to be evaporated is provided with electric power selected such that the metal oxide layers precipitated on the substrates to be coated are deposited at a precipitation rate of ≧4 nm/s. During the coating process the substrate to be coated is arranged stationary in relation to the target material to be evaporated. The electrodes are connected in a conductive manner to the outputs of an alternating current source whereby the alternating frequency of the alternating current provided for the electrical supply of the plasma discharge is selected between 10 kHz and 80 kHz. Particularly preferred is that the precipitated oxide layer is a TiO 2  layer or an SiO 2  layer.

FIELD OF THE INVENTION

[0001] The invention relates to a method for sputter-inducedprecipitation of metal oxide layers on substrates by means of a reactivesputter process and also to a layer system produced according to such amethod.

DESCRIPTION OF THE PRIOR ART

[0002] Methods of manufacture of this kind and single-layer systemsmanufactured according to this process are known. By means of a sputterdevice as described for example in U.S. Pat. No. DE 4,106,770,substrates are coated by means of cathode sputtering, preferably bymagnetron cathode sputtering, where so-called targets are exposed to theaction of a plasma cloud which forms between two electrodes, and wherethe evaporated target material which shows affinity to the reactive gas,forms a chemical bond with the gas and precipitates upon the substrate.In the known sputter methods the target is for example the one electrodeand the substrate the other electrode which is connected electrically toboth outputs of an electric power supply device. As an alternative, socalled double electrodes are also used, which electrodes are alternatelyswitched as anode and cathode. Electric power is supplied to theelectrodes either as direct current or as alternating current, asdescribed for example in DE OS 3,802,852.

[0003] For precipitation of dielectric layers such as for example SiO₂,Al₂O₃, ZrO₂, TiO₂, ZnO₂, the target to be evaporated is composed of thecorresponding metal components present in the above compounds and isacted upon by the plasma cloud composed for example of Ar/O₂ or Ar/N₂mix, which action evaporates the metal target.

[0004] A problem long known in the use of the generic sputter methodshas been to provide homogenous and uniformly composed layers of highoptical quality reproducible for industrial use. Thus dielectric layerssuch as for example SiO₂, Al₂O₃, or especially TiO₂, deposited forexample by the DC sputter method on glass surfaces or opticalcomponents, have shown less than the desired resistance to environmentalconditions, such as for example humidity acting upon the layers. Sputterand coating techniques realized by means of the DC sputter methodadditionally cause in a disadvantageous manner long and thereforeexpensive coating times during which the process parameters defining thecoating process must be held constant. In addition, the opticalproperties of layer systems produced using conventional sputter methodshave proved inadequate increasingly exacting requirements.

SUMMARY OF THE INVENTION

[0005] An object of the present invention is to provide a method ofsputter-induced production of metal oxide layers on substrates by meansof a reactive sputter process through which metal oxide layers of highoptical quality can be made available for industrial manufacture in areproducible and cost-efficient manner.

[0006] According to the invention, the object is accomplished by amethod of this kind as mentioned in the introduction according to claim1 in that electric power is applied to the plasma charge acting upon thesputter target to be evaporated by means of at least two electrodesarranged in the vicinity of each other in the plasma reaction space, andwhere electric power is selected such that oxide layers to beprecipitated on the substrate to be coated are deposited at a coatingrate of >4 m/s, whereby during the coating process the substrate to becoated is arranged stationary in relation to the target material to beevaporated. A coating rate of >40 nm m/min is proposed for substrateswhich during the coating process are to be moved in front of the sputtertarget as in so-called continuous systems. Metal oxide layers producedaccording to the method characteristics of claim 1 or claim 2 exhibit,surprisingly enough, several advantages vis-à-vis metal oxide layersproduced by conventional sputtering. Thus it was found that TiO₂ layersproduced according to the invention had a refractive index n between2.55 and 2.60. Conventional DC technique only produced n values between2.35 and 2.45. Metal oxide layers having a high n value advantageouslyallow a thinner metal oxide layer than one produced by conventionalmethods in order to achieve an effect dependent on the refractory value.In addition, thinner metal oxide layers have the advantage of high lighttransmission and color neutrality in the visible spectrum. Moreover,thin metal oxide layers can be produced more cost-effectively thanconventional metal oxide layers.

[0007] Layers produced according to the invention also advantageouslyexhibit a very smooth surface, as indicated in claim 5. Surfacestructure morphology of metal oxide layers produced according to theinvention exhibits a very compact crystalline definition whichdemonstrates high resistance to chemically reactive substances. Metaloxide layers produced according to the invention are correspondinglymore resistant to the effects of humidity than conventional sputterlayers produced for example by means of a plasma discharge. Furthermoreit was found that by using sputter plasma operated with alternatingcurrent the precipitated TiO₂ layers crystallized primarily in a rutilestructure. Contrary to the anatase structure of the TiO₂ layer whichprimarily forms in DC sputter process, the rutile structure istemperature-stable up to 1855° C., while the anatase structure undergoesa phase conversion at 642° C. and exhibits an unstable structure. It hasbeen further shown that given equal plasma output, the process accordingto the invention achieves a sputter rate about 6 to 7 times higher thanthat of conventional DC sputter process.

[0008] Metal oxide layers produced according to the invention can alsobe used for improved low-E layer systems and for so-called solar-controlsystems with improved optical properties. In the case of low E layersthe possibility is created advantageously to forgo tin as the targetmaterial in favor of economical titanium. Since SnO₂ layers produced bythe sputter process disadvantageously tend to develop islands, it isdesirable to substitute TiO₂ for SnO₂. In the case of low-E layersystems the base layer applied directly on the substrate exhibitsaccording to the invention a smooth and compact surface structure ontowhich the actual low E layer is applied, for example a silver or goldlayer. Morphology of the base layer produced according to the inventionalso advantageously promotes the forming of an applied metal layer whichexhibits high conductivity or a low k value, respectively.

[0009] It has been shown to be advantageous for the production of layersaccording to the invention to select a frequency of the alternatingcurrent supplying the sputter plasma between 10 kHz and 80 kHz, asindicated in claim 8.

[0010] Additional advantageous features of the method and possible usesof metal oxide layers according to the invention are characterized inmore detail in the subclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] The invention includes various possible embodiments. Severalespecially advantageous exemplary emdodiments are represented in thedrawings and are described in more detail below.

[0012] There are shown in

[0013]FIG. 1a Scanning electron microscope image of the surface of aTiO₂ layer produced according to prior art, viewed at an angle;

[0014]FIG. 1b enlarged portion of the TiO₂ layer shown in 1 a;

[0015]FIG. 2 an X-ray diffraction spectrum of the TiO₂ layer shown in 1a, 1 b;

[0016]FIG. 3a a scanning microscope image of a TiO₂ layer applied at adynamic coating rate of 21 nm m/min and a viewing angle of 60° inrelation to the normal of the surface;

[0017]FIG. 3b a scanning microscope image of a cross-section of the TiO₂layer shown in 3 a;

[0018]FIG. 4 an X-ray diffraction spectrum of the TiO₂ layer shown in 3a, 3 b;

[0019]FIG. 5a a scanning microscope image of a TiO₂ layer applied at adynamic coating rate of 37 nm m/min;

[0020]FIG. 5b a scanning microscope image of a cross-section of the TiO₂layer shown in 5 a;

[0021]FIG. 6 an X-ray diffraction spectrum of the TiO₂ layer shown in 5a, 5 b;

[0022]FIG. 7a a scanning microscope image of a TiO₂ layer applied byreactive sputter at a layer growth rate of 49 nm m/min;

[0023]FIG. 7b a scanning microscope image of a cross-section of the TiO₂layer shown in 7;

[0024]FIG. 8 an X-ray diffraction spectrum of the TiO₂ layer shown in 7a, 7 b;

[0025]FIG. 9 the characteristic curve of a high output DC cathode;

[0026]FIG. 10 the characteristic curve of a dual magnetron cathode and

[0027]FIG. 11 output curve and sputter rate measured as a function ofthe probe voltage for a sputter process according to the invention.

DETAILED DESCRIPTION

[0028] A TiO₂ layer 4, 6 produced by conventional DC sputter method inshown in FIGS. 1a and 1 b in various enlargements of a scanningmicroscope image. The TiO₂ layer is applied on an Si wafer 2 andexhibits a layer thickness of approximately 500 nm. The TiO₂ layer 4,6grown by means of reactive DC magnetron sputtering with addition of anAr/O₂ gas mixture consists of single columnar microcrystallites whichare arranged substantially parallel to each other on the substrate. Thesurface of the TiO₂ layer 4, 6 possesses a pronounced surface roughness.The X-ray crystallographic analysis of the TiO₂ layer 4, 6 shown inFIGS. 1a, 1 b is represented as a Debye-Scherer diagram in FIG. 2.Diffraction reflexes A₁, A₂ and A₃ which dominate in FIG. 2 arecharacteristic of the anatase structure of TiO₂. Accordingly, reflex A₁represents grid net planes assigned to the anatase structure 101 andfulfilling the Bragg reflexion conditions, reflex A₂ those of anatasestructure 004, and reflex A₃ those of anatase structure 112.

[0029] A TiO₂ layer produced by means of a reactive sputter process witha coating rate of 21 nm m/sec as proposed according to the invention isshown in FIGS. 3a and 3 b. TiO₂ layer 14 (see FIG. 3b) has a thicknessof about 500 nm and in comparison to the layer structure shown in FIG.1b, exhibits only weakly defined and locally limited columnar TiO₂microcrystallites. Surface 16 shown in FIG. 3a exhibits in placessurface sites which have only a small depth of roughness. Thecrystalline composition of TiO₂ layer 14 shown in FIGS. 3a, 3 b, appliedto a glass substrate, is evident in the Debye-Scherrer diagram (FIG. 4),which, beside the known anatase 101 structure (A₁), also shows thediffraction reflex R₁, which corresponds to the Bragg reflection in a110 grid net plane of a TiO₂ layer crystallized in a rutile structure.The rutile structure therein corresponds to the areas of low surfaceroughness appearing in FIG. 3a, while in contrast, the anatase 110structure corresponds to the island formations appearing in FIG. 3a.

[0030] A TiO₂ layer 24, 26, applied to a glass substrate with aapplication rate of 37 nm m/min is shown in FIG. 5a. In comparison toTiO₂ surfaces 14, 4, 6 shown in FIG. 3a and FIG. 1a, only occasionalmicro-crystalline island formations appear with the higher coating rate.Column formations corresponding to the micro-crystalline islandformations are no longer present in the cross-sectional view shown inFIG. 5b. This result is also confirmed by the corresponding X-raydiffraction spectrum (see FIG. 6). Accordingly, in the correspondingI(2θ) diagram, the rutile structure clearly dominates vis-à-vis thecompeting anatase 101 structure.

[0031] An even stronger definition of the rutile structure can be seenin FIGS. 7a and 7 b. The TiO₂ layer, with a thickness of approximately500 nm, applied on a glass substrate, is composed almost entirely ofrutile structures, as can be deduced from the corresponding X-raydiffraction spectrum in FIG. 8 on the basis of the rutile 110 reflexwhich is the only one present. Surface 36 of the TiO₂ layer, applied ata rate of 49 nm m/min, is nearly homogeneously smooth and exhibits onlyoccasional island formations 38. Microcrystalline anatase structureswhich form columns during growth are no longer visible in thecross-section shown in FIG. 7b.

[0032] The substantial difference between reactive sputter processaccording to prior art and the proposed sputter process according to theinvention follows from the comparison of the course of the respectivecharacteristic curves of the cathode of a DC sputter process (see FIG.9) and a AC sputter process (FIG. 10). FIG. 9 shows the characteristiccurve of a high output cathode supplied with direct current, into whichcathode a titanium target is integrated. What is shown is the totalpressure in the sputter chamber as a function of the O₂ gas volume Mflowing into the plasma reaction space. The p(M) curve exhibits ahysteresis loop between working points M₁ and M₂. Two sputter conditionsfor the DC cathode are possible in the transitional area between thepoints M₁ and M₂, i.e. a metallic one which will proceed along route W₁,and an oxide sputter condition which is present if following thehysteresis loop in the direction W₂. A cathode supplied with directcurrent uncontrollably jumps between the two modes in the transitionalarea between the points M₁ and M₂. Due to the broad transitional area ofthe hysteresis loop of the p(M) curve shown in FIG. 9, stable sputterconditions needed for high-quality metal oxide layers can only beachieved with the help of expensive process control devices. To achievemaximum sputter rates with a high output DC cathode, a value of theoxygen volume supply is sought in the M₁ area, for the selection of thesputter working point, where the metal oxide layers can be produced inthe oxide mode.

[0033]FIG. 10 shows the characteristic curve of a high output magnetroncathode supplied with alternating current, according to the invention.The figure clearly shows that the transitional area between the workingpoints M₃ and M₄ has a width of only 10 sccm and is therefore narrowerby a factor of 7 than in the case of conventional sputter process (seeFIG. 9):

[0034]FIG. 11 shows measured sputter rates Y_(sp) and cathode output Lat a constant oxygen supply of 120 sccm as a function of probe voltageU_(s) for a sputter process according to the invention. According to themeasuring process a higher probe voltage U_(s) corresponds to a loweroxygen proportion in the sputter chamber. It can be observed that thehigher the probe voltage U_(s), the more metallic the burning of thecathode. The sputter rate rises advantageously with falling oxygenvolume and at the same time the cathode output falls (see curve run A).An oxygen sensor serves for the regulation of the sputter process, whichsensor provides the probe voltage U_(s) as an actual value assigned to acontrol circuit. FIG. 11 shows that rising sputter rates Y_(sp) arerealized with a smaller cathode output L. This significantly increasesthe efficiency of the sputter arrangement according to the invention.

What is claimed is:
 1. Process for sputter-induced precipitation ofmetal oxide layers on substrates by means of a reactive sputter process,characterized in that the plasma charge acting upon the target to beevaporated is provided with electric power selected such that the metaloxide layers precipitated on the substrates to be coated are depositedat a precipitation rate of ≧4 nm/s, whereby during the coating processthe substrate to be coated is arranged stationary in relation to thetarget material to be evaporated.
 2. Process for sputter-inducedprecipitation of metal oxide layers on substrates by means of a reactivesputter process, characterized in that the oxide layers to beprecipitated on the substrate to be coated are deposited at aprecipitation rate of ≧40 nm m/min, whereby the substrate to be coatedis moved along in front of the target material to be evaporated. 3.Process according to claim 1 or 2 , characterized in that the electrodesare connected in a conductive manner to the outputs of an alternatingcurrent source whereby the alternating frequency of the alternatingcurrent provided for the electrical supply of the plasma discharge isselected between 10 kHz and 80 kHz.
 4. Process according to at least oneof the claims 1 to 3 , characterized in that the precipitated oxidelayer is a TiO₂ layer or an SiO₂ layer.
 5. Optical-effect layer system,arranged on substrate surfaces and produced according to at least one ofthe preceding claims, which layer system exhibits an order of layerscomposed of alternating low refractive and high refractive layers, wherethe individual oxide layers are precipitated on the substrate surfacesto be coated by means of a sputter-induced evaporation and precipitationprocess carried out in a vacuum chamber and where the sputter plasma issupplied by alternating current fed to the plasma electrodes,characterized in that the predominant part of the oxide layer exhibits arutile structure.
 6. Layer system according to claim 5 , characterizedin that the metal oxide layers are deposited at a precipitation rateof >4 nm/s on the target which is arranged stationary in relation to thesputter cathode.
 7. Layer system according to claim 5 , characterized inthat the oxide layer is an SiO₂ layer which is deposited at aprecipitation rate of >5 nm/s on the substrate which is arrangedstationary, preferably spatially secure in relation to the sputtercathode.
 8. Layer system according to at least one of the claims 5through 8, characterized in that the alternating frequency of thealternating current fed to the sputter electrode lies between 10 kHz and80 kHz.
 9. A process for sputter-induced precipitation of metal oxidelayers on substrates by means of a reactive sputter process, saidprocess comprising: providing a target to be evaporated and creating aplasma charge thereabout; providing electric power to the plasma chargeacting upon the target to be evaporated, said electric power beingselected such that the metal oxide layers precipitated on the substrateto be coated are deposited at a precipitation rate greater than or equalto 4 nm/s; and maintaining the substrate to be coated in stationaryrelation to the target material to be evaporated during the coatingprocess.
 10. A process for sputter-induced precipitation of metal oxidelayers on substrates by means of a reactive sputter process, saidprocess comprising: providing a target material for said sputterprocess; depositing the oxide layers to be precipitated on the substrateto be coated at a precipitation rate greater than or equal to 40 nmm/min; and moving the substrate to be coated along in front of thetarget material to be evaporated.
 11. Process according to claim 9 ,wherein electrodes are connected in a conductive manner to outputs of analternating current source whereby the alternating frequency of thealternating current provided for the electrical supply of the plasmadischarge is selected between 10 kHz and 80 kHz.
 12. Process accordingto claim 10 , wherein electrodes are connected in a conductive manner tooutputs of an alternating current source whereby the alternatingfrequency of the alternating current provided for the electrical supplyof the plasma discharge is selected between 10 kHz and 80 kHz. 13.Process according to claim 9 , wherein the precipitated oxide layer is aTiO₂ layer or an SiO₂ layer.
 14. Process according to claim 10 , whereinthe precipitated oxide layer is a TiO₂ layer or an SiO₂ layer. 15.Process according to claim 11 , wherein the precipitated oxide layer isa TiO₂ layer or an SiO₂ layer.
 16. Process according to claim 12 ,wherein the precipitated oxide layer is a TiO₂ layer or an SiO₂ layer.17. An optical-effect layer system comprising: a substrate having asurface; a plurality of oxide layers applied successively over saidsurface according to the process of claim 9 ; the layers being appliedto the substrate surface in an order such that the layers alternatebetween being low refractive layers and high refractive layers; theindividual oxide layers being precipitated on the substrate surface bymeans of a sputter-induced evaporation and precipitation process carriedout in a vacuum chamber, and wherein sputter plasma is suppliedalternating current fed to plasma electrodes; the oxide layer having apredominant part having a rutile structure.
 18. A layer system accordingto claim 17 , wherein the metal oxide layers are deposited at aprecipitation rate greater than 4 nm/s on the target which is arrangedstationary in relation to the sputter cathode.
 19. A layer systemaccording to claim 17 , wherein the oxide layer is an SiO₂ layer whichis deposited at a precipitation rate greater than 5 nm/s on thesubstrate which is arranged stationary in relation to the sputtercathode.
 20. A layer system according to claim 17 , wherein thealternating frequency of the alternating current fed to the sputterelectrode lies between 10 kHz and 80 kHz.
 21. An optical-effect layersystem comprising: a substrate having a surface; a plurality of oxidelayers applied successively over said surface according to the processof claim 10 ; the layers being applied to the substrate surface in anorder such that the layers alternate between being low refractive layersand high refractive layers; the individual oxide layers beingprecipitated on the substrate surface by means of a sputter-inducedevaporation and precipitation process carried out in a vacuum chamber,and wherein sputter plasma is supplied alternating current fed to plasmaelectrodes; the oxide layer having a predominant part having a rutilestructure.